UPDATE: The biggest problem I had with these in the field is that the arm that wraps around to hold the vertical leg is too long and makes the legs too closely spaced. I have shortened these arms in V7, to give the legs a wider stance, but have not printed them yet to try. I also added a longer "extended" version, per a request, though I think these are significantly longer than is practical.
I am still trying to get a functional leg extension for my new Phantom 4 Pro. This is an updated version of my previous attempt (https://www.thingiverse.com/thing:2897392).
These legs are a little more complex, they still snap on to the bottom of the phantom 4 legs, but now they have an arm that wraps around and holds on to the vertical legs to keep the extensions from rotating if it gets some non-perpendicular force during landing. I retained 2x holes for small zip ties, but the goal was to be able to use these as snap on/snap off parts so they can be easily removed for transport.
This remains a work in progress. I have just finished printing the first successful set of 4. They are not trivial to print, since they need some support to get the arm extensions right. The first set prints well, but I probably want to make the arms a little shorter so the extensions sit a little further out and provide a wider base.
I only include 1 version here, so you need to mirror these correctly in your slicer so the arms all wrap the right way.